on 28-Apr-2014 06:16
To get the APM Cookbook series moving along, I’ve decided to help out by documenting the common APM solutions I help customers and partners with on a regular basis.
Kerberos SSO is nothing new, but seems to stump people who have never used Kerberos before. Getting Kerberos SSO to work with APM is straight forward once you have the Active Directory components configured.
I have a pre-configured web service (IIS 7.5/Sharepoint 2010) that is configured for Windows Authentication, which will send a “Negotiate” in the header of the “401 Request for Authorization”. Make sure the web service is configured to send the correct header before starting the APM configuration by accessing the website directly and viewing the headers using browser tools.
In my example, I used the Sharepoint 2010/2013 iApp to build the LTM configuration. I’m using a single pool member, sp1.f5.demo (10.10.30.2) listening on HTTP and the Virtual Server listening on HTTPS performing SSL offload.
1.1 Open Active Directory Users and Computers administrative tool and create a new user account.
1.2 Alter the account and set the servicePrincipcalName. Run setspn from the command line:
setspn –A host/apm-kcd.f5.demo apm-kcd
A delegation tab will now be available for this user.
2.1 Open Active Directory Users and Computers administrative tool and select the user account created in the previous step.
2.2 Make sure there are no duplicate SPNs and run setspn –x from the command line.
DNS is critical and a missing PTR is common error I find when troubleshooting Kerberos SSO problems.
From the BIG-IP command line test forward and reverse records exist for the web service using dig:
# dig sp1.f5.demo
;; QUESTION SECTION:
;sp1.f5.demo. IN A
;; ANSWER SECTION:
sp1.f5.demo. 1200 IN A 10.10.30.2
# dig -x 10.10.30.2
;; QUESTION SECTION:
;2.30.10.10.in-addr.arpa. IN PTR
;; ANSWER SECTION:
2.30.10.10.in-addr.arpa. 1200 IN PTR sp1.f5.demo.
In this example I will use a Logon Page to capture the user credentials that will be authenticated against Active Directory and mapped to the SSO variables for the Kerberos SSO.
4.1 Configure AAA Server for Authentication
Access Policy >> AAA Servers >> Active Directory >> “Create”
Supply the following:
Select “Finished" to save.
4.2 Configure Kerberos SSO
Access Policy >> SSO Configurations >> Kerberos >> “Create”
Supply the following:
Select “Finished” to save.
4.3 Create an Access Profile and Policy
We can now bring it all together using the Visual Policy Editor (VPE).
Access Policy >> Access Profiles >> Access Profile List >> “Create”
Supply the following:
Select “Finished” to save.
4.4 Edit the Access Policy in the VPE
Access Policy >> Access Profiles >> Access Profile List >> “Edit” (intranet.f5.demo_sso_ap)
On the fallback branch after the Start object, add a Logon Page object.
On the fallback branch after the Logon Page object, add an AD Auth object.
On the Successful branch after the AD Auth object, add a SSO Credential Mapping object.
On the fallback branch after the SSO Credential Mapping, change Deny ending to Allow. The finished policy should look similar to this:
Don't forget to “Apply Access Policy”.
5.1 Edit the Virtual Server
Local Traffic >> Virtual Servers >> Virtual Server List >> intranet.f5.demo_vs
Scroll down to the Access Policy section and select the Access Profile.
Select “Update” to save.
5.2 Test
Open a browser, access the Virtual Server URL (https://intranet.f5.demo in my example), authenticate and verify the client is automatically logged on (SSO) to the web service. To verify Kerberos SSO has worked correctly, check /var/log/apm on APM by turning on debug. You should see log events similar to the ones below when the BIG-IP has fetched a Kerberos Ticket.
info websso.1[9041]: 014d0011:6: 33186a8c: Websso Kerberos authentication for user 'test.user' using config '/Common/f5.demo_kerberos_sso'
debug websso.1[9041]: 014d0018:7: sid:33186a8c ctx:0x917e4a0 server address = ::ffff:10.10.30.2
debug websso.1[9041]: 014d0021:7: sid:33186a8c ctx:0x917e4a0 SPN = HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0023:7: S4U ======> ctx: 33186a8c, sid: 0x917e4a0, user: test.user@F5.DEMO, SPN: HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0001:7: Getting UCC:test.user@F5.DEMO@F5.DEMO, lifetime:36000
debug websso.1[9041]: 014d0001:7: fetched new TGT, total active TGTs:1
debug websso.1[9041]: 014d0001:7: TGT: client=apm-kcd@F5.DEMO server=krbtgt/F5.DEMO@F5.DEMO expiration=Tue Apr 29 08:33:42 2014 flags=40600000
debug websso.1[9041]: 014d0001:7: TGT expires:1398724422 CC count:0
debug websso.1[9041]: 014d0001:7: Initialized UCC:test.user@F5.DEMO@F5.DEMO, lifetime:36000 kcc:0x92601e8
debug websso.1[9041]: 014d0001:7: UCCmap.size = 1, UCClist.size = 1
debug websso.1[9041]: 014d0001:7: S4U ======> - NO cached S4U2Proxy ticket for user: test.user@F5.DEMO server: HTTP/sp1.f5.demo@F5.DEMO - trying to fetch
debug websso.1[9041]: 014d0001:7: S4U ======> - NO cached S4U2Self ticket for user: test.user@F5.DEMO - trying to fetch
debug websso.1[9041]: 014d0001:7: S4U ======> - fetched S4U2Self ticket for user: test.user@F5.DEMO
debug websso.1[9041]: 014d0001:7: S4U ======> trying to fetch S4U2Proxy ticket for user: test.user@F5.DEMO server: HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0001:7: S4U ======> fetched S4U2Proxy ticket for user: test.user@F5.DEMO server: HTTP/sp1.f5.demo@F5.DEMO
debug websso.1[9041]: 014d0001:7: S4U ======> OK!
Like I said in the beginning, once you know how Kerberos SSO works with APM, it’s a piece of cake!
userPrincipalName=%{session.logon.last.username}
That will retrieve the entire AD object for the user that has authenticated via Kerberos, and you will be able to use samAccountName attribute in your SAML assertion(which will effectively have their account name without the domain in it.
A few updates.
In Section 4.2:
KDC - Leave Blank, but be aware of https://support.f5.com/kb/en-us/solutions/public/16000/400/sol16483.html
Account Name: use host/apm- instead of the sAMAccountName
hi Bret,
Great guide ! could you please detail the last step in 2.1, it says "Enter the host name, in my example I will be adding HTTP service for sp1. (SP1)."
So what exactly is this SP1? is it a user you created earlier or what
Thanks a lot
Looking at your guide, and Kerberos SSO, for the first time. I haven't seen AD account creation with an account starting with a slash, please explain. Am I missing something too obvious?
What is the significance of "host/" in the account name? Should "host/" be the first part of the account name? Is "host" representative of another value or is this literal?
Hi The-messenger,
The user logon name needs to be in the Service Principal Name (SPN) form: https://msdn.microsoft.com/en-us/library/ms677949(v=vs.85).aspx
Cheers,
Brett
Most intranet sites don't require you to type in credentials. Won't the "Logon Page" prompt for creds? I thought the whole point of kerberos was to provide seamless authentication. Guess I'm missing something.
Roger that. We have an internal site that has seamless auth. When we put it behind the F5 the auth broke. Using APM, how would I continue the seamless auth? Using chrome tools I see:
Server response:
HTTP/1.1 401 Unauthorized
Content-Type: text/html
Server:
WWW-Authenticate: Negotiate
WWW-Authenticate: NTLM
Client reply:
Authorization:Negotiate YIIMjgYGKwYBBQ....
From what I've read the "Y" in the client reply indicates kerberos. Can you provide a link to guide me in the right direction? From what I've seen you can start the VPE off with a "Client Cert Inspection" or a "HTTP 401 Response". I believe we have the ability to do both. Which one is better. Thanks for your time.
Hi RiverFish,
If this is an internal site check your SPN and IIS is configured correctly: https://blogs.technet.microsoft.com/askds/2011/08/09/kerberos-and-load-balancing/
No APM required.
Cheers,
Brett
Sounds like you need to set the "SPN Pattern" on the Kerberos SSO configuration. From the APM help page:
 
Also have a look at: https://devcentral.f5.com/s/feed/0D51T00006i7ZhkSAE
 
Hi, this works perfectly. What if i need a client to do the kerberos auth and the APM do the krb SSO. Can you describe how to configure it correctly Brett?
Hi Martin,
Best place to start is here: https://support.f5.com/kb/en-us/products/big-ip_apm/manuals/product/apm-authentication-single-sign-o...
You need to configure Kerberos Auth on the Client side and KCD SSO (this article) for server side.
Sounds like I need to write another article with a Kerberos Auth example when I get time. 🙂
Hi Brett, thanks, I think I've done the AAA krb auth right.
Ending up with this
Oct 17 15:54:52 sok5-f5 err websso.3[23639]: 014d0026:3: /Common/pptest2:Common:61e5157b: Could not find SSO username, check SSO credential mapping agent setting
Oct 17 15:54:52 sok5-f5 debug websso.3[23639]: 014d0041:7: /Common/pptest2:Common:61e5157b: Could not find SSO domain, using default realm defined for Kerberos SSO object
Oct 17 15:54:52 sok5-f5 err websso.3[23639]: 014d0043:3: /Common/pptest2:Common:61e5157b: SSO username is empty - SSO is disabled
I can see the authenticated session. Variable session.logon.last.username by AAA is something like this user@KRB-REALM.ORG. Is the right thought that this should be mapped to be used by SSO (backend krb delegation) to defined variables (session.sso.token.last.username and session.ad.last.actualdomain in this scenario) by SSO credentail mapping? Can you advise me me how to achieve this?
Hi Martin,
You can split the user@KRB-REALM.ORG (session.logon.last.username) into seperate variables using TCL or add an AD Query object to the VPE and set the "SearchFilter" to "userPrincipalName=%{session.logon.last.username}". It will populate "session.ad.last.attr.sAMAccountName" and "session.ad.last.actualdomain". Then change the "SSO Credential Mapping" object to use "session.ad.last.attr.sAMAccountName" instead of "session.logon.last.username".
Cheers,
Brett
I have chosen the TCL way. AD query consumes time 🙂
Variable assign contains:
session.sso.token.last.username = expr { [regsub {(.+\|^)} [mcget {session.logon.last.username}] "" x; set username $x; regsub {(\@.+)} $username "" y; set username $y]}
session.ad.last.actualdomain = expr { [regsub {(\.+)} [mcget {session.logon.last.username}] "" x; set username $x; regsub {(.+\@)} $username "" y; set username $y]}
This splits the krb username@realm into two variables
Hi - i am having the same issue - F5 logging this Message
Could not find SSO domain, using default realm defined for Kerberos SSO object
How did you resolve this issue ?
Thanks
I followed this article and for me after I browse to the virtual server and submitted the credentials in the login form (username/password), on backend, it is indeed hitting the pool member, but on the user side, it is prompting me with another credential popup for username/password.
NTP is fine.
time is in sync on all the three dog heads. (client, server, KDC).
DNS A, PTR records are in place.
no duplicate SPN.
It was not working. Basically SSO is not triggering.
Finally I found the root cause. On the SSO object configuration for the username, article says to use "apm-kcd". But when I used it, it was not working. After changed it to host/apm-kcd.f5.demo SSO started working.